1. Field of the Invention
The present invention relates to computer data storage devices and, in particular, relates to a hard disk drive having an actuator coil that is inhibited from overheating and methods for providing the same.
2. Description of the Related Art
Hard disk drive storage devices are an important component in virtually all computer systems. In particular, hard disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost. However, because the computer industry continually strives to provide computer systems with increased performance, there exists a need for improved disk drives having increased data access speeds.
The typical hard disk drive comprises one or more pivotally mounted disks having a magnetic recording layer disposed thereon and a plurality of magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are grouped into concentric circular tracks each having a unique radius on the disk and data is written to or read from each track by positioning the transducer adjacent the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
To position the transducer with respect to the disk, the typical hard disk drive further comprises a pivotally mounted actuator arm for supporting the transducer, a voice coil motor (VCM) for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM comprises a coil of conducting wire wound into a plurality of loops and a permanent magnet disposed adjacent the coil. The servo-controller initiates movement of the actuator arm by directing a control current to flow through the coil which results in the permanent magnet applying a force onto the coil which is then transferred to the actuator arm in the form of a torque. Because the direction of the torque is dictated by the direction of control current flow, the servo-controller is able to reposition the transducer by first directing the control current through the coil so as to angularly accelerate the actuator arm in a first direction and then reversing the control current so as to angularly decelerate the actuator arm.
The time required to reposition the transducer in the foregoing manner is known as the xe2x80x9cseek timexe2x80x9d of the drive and is an important performance factor that limits the throughput of the drive. For example, a drive having a short seek time will be able to access a requested track of data more quickly than a drive having a longer seek time. According to the state of the art, the seek time required to reposition the transducer across a distance of 2-5 cm is typically in the range of 5-10 ms, thereby resulting in the transducer having a linear acceleration greater than 500 m/s2 or 50 g""s. Consequently, to provide such large acceleration, a relatively large current is often required to flow through the coil.
Unfortunately, when large amounts of current are directed through the coil, the rate of heat gain caused by the finite resistance of the windings of the coil may exceed the rate of heat loss to the environment. Thus, if left unchecked for an extended period of time, a rapid succession of seek operations may excessively raise the temperature of the coil such that the drive will no longer be operable. For example, when subjected to an instantaneous or average current that is beyond the VCM""s design limitations, the coil may generate excessive heat and deform the coil. Moreover, overmold material may delaminate from the actuator assembly, lose its rigidity and/or outgas particulates into the disk drive enclosure, with deleterious results. Such outgassing from the coil overmold, coil insulators and/or from other materials applied to the coil wires (such as wire lubricants, for example) may occur even at relatively low temperatures (85xc2x0 C., for example). Thus, to prevent such damage, there is a need to inhibit VCM coil from overheating.
One possible solution to the problem of excessive coil temperature is to blindly limit the VCM control current, i.e. without sensing or estimating the coil temperature, so as to be absolutely sure that the temperature of the coil is less than a threshold value. For example, following a first seek operation, a subsequent seek could be delayed so as to be sure that heat added to the coil during the first seek operation is substantially dissipated to the environment before the subsequent seek occurs. Alternatively, the resistive heat gain in the coil could be reduced by reducing the commanded current through the coil. However, because of the difficulty in estimating how well the environment can remove heat from the coil, the foregoing methods of blindly limiting the coil current will likely require using overly conservative limitations. Thus, while possibly preventing the coil from overheating, the foregoing solution can result in unacceptably slow drive performance.
Another solution is proposed in U.S. Pat. No. 5,594,603 to Mori et al. and assigned to Fujitsu Limited, Japan. In this patent, the current applied to the coil is used to approximate the coil temperature. This method attempts to mathematically model the thermal behavior of the coil by inter-relating a group of factors that includes the VCM control current, the heat naturally radiated by the coil, the ambient temperature, and the thermal capacity of the coil. However, such modeling, although providing an indication of the present VCM temperature, requires considerable processing resources, thereby requiring the drive to include more expensive data processing components.
To reduce costs, the typical disk drive includes only a single processor. Usually, the single processor is required to perform many different functions, such as communicating with a host computer, keeping track of where data is stored on the disk, and controlling the movement of the transducer elements. Thus, the processor is xe2x80x9cbandwidth limitedxe2x80x9d such that the processor is usually busy and, therefore, unable to perform the relatively aforementioned complicated calculations of the prior art.
Another proposed solution is proposed in U.S. Pat. No. 5,128,813 to Lee (hereafter the ""813 patent) and assigned to Quantum Corporation. In this patent, a discrete temperature-sensing element is used to dynamically sense the VCM temperature during the operation of the drive. This patent discloses that the thermistor is mounted for thermal conduction directly to the head and disk assembly. While the temperature sensing element may, in fact, provide a direct measurement of the temperature of the VCM (in contrast to the Mori et al. patent above, for example), this method requires mounting a high precision thermistor to the drive and requires that appropriate signal conditioning means be provided to measure, quantize and interpret the resistance of the thermistor. In many aspects, however, disk drive designers and manufacturers operate in an environment that has acquired many of the characteristics of a commodity market. In such a market, the addition of even a single, inexpensive part can directly and adversely affect competitiveness. In this case, therefore, the addition of the thermistor and associated signal conditioning means discussed in the ""813 patent may be of little practical value.
From the foregoing, it will be appreciated that there is a need for improved methods of inhibiting a voice coil motor from overheating that are inexpensive in their implantation and do not require substantially increased processing resources.
The aforementioned needs are satisfied by the hard disk drive and the method of operating a hard disk drive of the present invention. In one aspect, the present invention comprises a method of inhibiting a voice coil of a hard disk drive from overheating, the method comprising performing a plurality of seek operations, and adjusting a value stored in a register by amounts which are indicative of the heat which is produced in the voice coil during the plurality of seek operations. The method further comprises counter-adjusting the values stored in the registers so as to oppose the adjusting, the counter-adjusting occurring at a rate which is determined by a substantially periodic signal. The method also comprises modifying seek operations when the value of the register is outside of the preferred range. In this particular aspect, a register of the control system of the drive can contain a running value which is indicative of the temperature of the drive as this value is being adjusted when seek operations are occurring, i.e., when heat is being added to the drive and this value is also be counter-adjusted at a rate which is based upon the rotational speed of the disk of the drive.
In one particular embodiment, the method further comprises adjusting a value in the register by adding an amount that is selected from a look-up table which is indicative of the heat being added to the coil for a particular seek operation. In one embodiment, the method also comprises counter-adjusting the value by decreasing the value in the register by a selected amount in response to a servo wedge of the disk passing a transducer of the drive. Hence, a value can be simply and efficiently stored in the register that is indicative of the heat that has been added to the drive and this value can also be adjusted downward based upon the elapsed time in which the drive has cooled wherein the elapsed time value is being provided by the periodic detection of servo wedges on the rotating disk.
In another aspect of the invention, the invention comprises a method of inhibiting a voice coil of a hard disk drive from overheating, the method comprising performing a plurality of seek operations so as to reposition a transducer adjacent a recording medium a plurality of times, wherein each seek operation adds heat to the coil. The method further comprises determining a plurality of thermally characterized values wherein each thermally characterized value is indicative of the heat that is added to the coil during a corresponding seek operation of the plurality of seek operations. The method further comprises accumulating the plurality of thermally characterized values in a register and periodically reducing the values stored in the register and modifying seek operations when the value of the register is greater than a threshold value. In this way, a running value can be maintained in a register which is indicative of the temperature of the coil in an efficient manner.
In one particular embodiment, the thermally characterized value is the estimate of the time period that is required for the heat added to the coil during the corresponding seek operation to dissipate away from the coil. In this embodiment, the thermally characterized values comprise a look-up table. In particular implementation of this embodiment, the look-up table is an X-Y look-up table that correlates to an estimated heat dissipating duration (Y) to seek length (X). In one particular implementation, the look-up table defines a positively sloped line that begins at a first point (X1, Y1) and extends to a second point (X2, Y2) and a horizontal line that extends from the second point. In this implementation, X2 is equated with a seek length of maximally heating seek operations such that the transducer is always accelerating and the transducer reaches a maximally allowed speed during the maximally heating seek operation. Moreover, in this particular implementation, Y2 is determined by performing a plurality of maximally heating seek operations on a substantially identical hard disk drive such that a delay period, xcex94T is introduced between successive seek operations so as to maintain the temperature of the substantially identical hard drive at a constant elevated value.
In yet another aspect of the present invention, the present invention comprises a hard disk drive comprising a magnetic medium, a transducer that interacts with a magnetic medium, an actuator comprising a voice coil that accelerates the transducer, and a control system. In this particular aspect, the control system includes a register and the control system instructs the actuator to perform a plurality of seek operations so as to reposition the transducer between a plurality of locations adjacent the magnetic medium. In this aspect, the register stores a value which is indicative of the thermal energy of the coil, and the control system adjusts the value of the register according to the plurality of seek operations so as to account for the heat that is added to the coil during the plurality of seek operations. Moreover, the control system repeatedly counter-adjusts the value of the register so as to account for heat that dissipates away from the coil. The control system further modifies subsequent seek operations when the value stored in the register is outside of a preferred range so as to reduce the rate at which heat is added to the coil.
In these particular aspects of the present invention, a process for determining and estimating the heat in a coil so as to inhibit damage to the coil as a result of successive seek operations can be performed efficiently. More particularly, the use of a register that is adjusted up and down based upon predetermined values allows for a more efficient and simply implemented process of estimating the heat within a coil which further allows the coil to be operated closer to the damage threshold of the coil. These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.